Newspapers are produced predominantly by wet offset. Printing presses to which the present invention preferably pertains typically contain printing units with rubber blanket cylinders, plate cylinders, inking units and damping units. A printing form mounted on a printing form cylinder has a surface mostly in the form of a top layer, which has hydrophilic (water-attracting) and lipophilic (water-repelling) areas in the illustrated state. The printing form is usually formed by a printing plate, which is mounted on a printing form cylinder designed as a plate cylinder. The printing form has lipophilic areas applied in the pattern of an image. The non-image areas arc hydrophilic and bind water more strongly than ink. The lipophilic areas repel water and therefore exert an ink-attracting action. In principle, any surface that can be divided into hydrophilic and lipophilic areas may be used for the offset process.
A large number of processes and devices using corresponding printing forms are known for producing printing forms. For example, a printing form can be irradiated with a laser in the pattern of an image and subsequently developed chemically. Furthermore, printing forms can be prepared by laser ablation. In this process, lipophilic areas are exposed under a hydrophilic layer or hydrophilic areas are exposed under a lipophilic layer. The exposure operation that is decisive for producing the image may be carried out either in a separate unit or within the printing press, as is preferred according to the present invention. The outer drum principle is known for exposure or illustration in the printing press. So-called process-free printing forms, which do not require any chemical development, are used in most cases.
The printing forms used currently are used only once. However, it is desirable for economic and ecological reasons to be able to use these printing forms several times.
The illustration of a printing form by a photocatalytic reaction is known from EP 0 911 155 A1. To produce the printing style, the hydrophilic non-image areas are irradiated with UV laser light. The printing form thus exposed and illustrated as a result is erased by heating. The printing form must reach a high temperature in the process. Furthermore, a cleaning means is needed for erasing of the printing style after the use of the printing form in order to remove the ink residues from the printing form. Without cleaning, heating of the printing form to erase the printing style would bring about the burning in of ink residues in the surface of the printing form, as a result of which the printing form would become unusable.
EP 0 911 154 A1 describes the illustration by heating in the pattern of an image and the erasing by UV irradiation. Further details are described in EP 1 020 304 A2.